The use of polymeric materials for medical devices is an area where vast improvements in polymeric materials have evolved and are still evolving. Physical properties of these polymers can be fine tuned for use in different environments or to behave in a predictable manner. For example, polymers for use in fabricating intraocular lenses (IOLs), need adaptation allowing for smaller incisions during implantation as well as attain a lower level of leachable content in the polymeric material itself.
Subsequently, IOLs were designed for smaller incisions through the use of elastomeric compositions that could be rolled or folded, inserted into the capsular sac and then unfolded once inside. Occasionally, folding of the IOL before insertion resulted in permanent deformation, which adversely affected the implant's optical qualities. Further, while foldable IOLs eliminated the need for the large incision, foldable IOLs were not without drawbacks. In particular, both non-deformable and foldable IOLs are susceptible to mechanical dislocation resulting in damage to the corneal endothelium.
Another approach to small incision IOL implantation uses an elastomeric polymer that becomes pliable when heated to body temperature or slightly above. Specifically, the IOL is made pliable and is deformed along at least one axis, reducing its size for subsequent insertion through a small incision. The IOL is then cooled to retain the modified shape. The cooled IOL is inserted into the capsular sac and the natural body temperature warms the IOL at which point it returns to its original shape. The primary drawback to this type of thermoplastic IOL is the limited number of polymers that meet the exacting needs of this approach. Most polymers are composed of polymethylacrylate which have solid-elastomeric transition temperatures above 100° C. Modifications to the polymer substrate require the use of plasticizers that may eventually leach into the eye causing harmful effects.
Dehydrated hydrogels have also been used with small incision techniques. Hydrogel IOLs are dehydrated before insertion and naturally rehydrated once inside the capsular sac. However, once fully rehydrated, the polymer structure is notoriously weak due to the large amount of water absorbed. The typical dehydrated hydrogel's diameter will expand from 3 mm to 6 mm resulting in an IOL that is 85% water. At this water concentration the refractive index (RI) drops to about 1.36, which is unacceptable for an IOL since lower RI materials require the optic to be thicker to achieve a given optical power.
Modern acrylate IOLs generally possess excellent mechanical properties such as foldability, tear resistance and physical strength. Acrylate IOLs also are known to possess good optical properties (transparency, high refractive index, etc.) and biocompatibility. While pure acrylic IOLs have desirable mechanical, optical and biological properties, they may have unacceptable molecular response times such that the folded or compacted IOL may not unfold as quickly as desired. A pure acrylate IOL fabricated to have a relatively fast molecular response time may be extremely tacky and lack the desired mechanical strength. In this case, the resulting IOL may tear and/or the resulting self-tack can render unfolding difficult.
Pure silicone IOLs generally possess excellent mechanical, optical and biological properties similar to pure acrylate IOLs. Unlike acrylic IOLs, silicone IOLs generally possess faster molecular response times. In fact, silicone IOLs may be so responsive that when folded small enough to be inserted through a 3 mm or smaller incision, the stored energy can cause the IOL to unfold more quickly than desired.
There is also a need for a polymeric material with a molecular response time which makes it suitable for use near fragile body tissues. There is also a need for ophthalmic devices in which one polymeric material is useful for both low modulus and high modulus applications to, inter alia, simplify the multi-part polymeric article manufacturing process and create better integrated multi-part polymeric articles in which the various parts have a common value of a property such as a refractive index.
Finally, there is a need for polymeric material that allows for a lower level of leachable material present in the lens for implantation. Presently, silicone polymeric materials used for optic soft gel implant applications suffer from an excess of leachable material resulting from a desire to keep the optic material from deforming during the manufacturing process. This presents a danger to the patient as the leachable material can begin to leach immediately upon implantation into the eye. Presently, to remedy this, multiple extraction steps are used ridding the IOL of leachables. However, each extraction causes lens material deformation and the need for remolding of the lens which comes at a great expense. Therefore, silicone materials need to be developed that allow multiple or long extraction steps without substantially deforming the lens material and which possess a substantially low leachable content when implanted into the eye.